观察者模式是一对多的一种模式，当有状态发生变化的时候，所有监听这个事件的对象都会得到通知并做相应的更新。此模式特别适合事件的监听，对该事件感兴趣的可以注册监听，当不需要的适合就取消注册，事件发生适合，广播通知所有注册对象。我们结合智能指针来实现一个简单的观察者模式，智能指针可以方便的判断注册对象是否还存在，可以有效的避免对象已经不存在了还调用

class IObject {
public:
    virtual void update() = 0;
};

class object_test1 : public IObject {
public:
    void update() {
        std::cout << "object_test1 update" << std::endl;
    }
};

class object_test2 : public IObject {
public:
    void update() {
        std::cout << "object_test2 update" << std::endl;
    }
};

class object_test3 : public IObject {
public:
    void update() {
        std::cout << "object_test3 update" << std::endl;
    }
};
class subject_test {
public:
    subject_test() {}
    ~subject_test() {}

public:
    void register_object(int id, std::weak_ptr<IObject> ob_ptr) {
        map_test_[id] = ob_ptr;
    }

    void unreg_object(int id) {
        auto it = map_test_.find(id);
        if (it != map_test_.end()) {
            map_test_.erase(it);
        }
    }

    void notify() {
        for (auto it = map_test_.begin(); it != map_test_.end(); ) {
            std::shared_ptr<IObject> sp = it->second.lock();
            if (sp) {
                sp->update();
                it++;
            } else {
                std::cout << "notify msg, but it's invalid, so erase it" << std::endl;
                it = map_test_.erase(it);
            }
        }
    }
private:
    std::map<int, std::weak_ptr<IObject> > map_test_;
};

int main()
{
    subject_test st;
    {
        std::shared_ptr<IObject> p1 = std::make_shared<object_test1>();
        std::shared_ptr<IObject> p2 = std::make_shared<object_test2>();
        std::shared_ptr<IObject> p3 = std::make_shared<object_test3>();
        st.register_object(1, std::weak_ptr<IObject>(p1));
        st.register_object(2, std::weak_ptr<IObject>(p2));
        st.register_object(3, std::weak_ptr<IObject>(p3));
        st.notify();
        st.unreg_object(1);
    }
    st.notify();
   
}

object_test1 update
object_test2 update
object_test3 update
notify msg, but it's invalid, so erase it
notify msg, but it's invalid, so erase it